Facile Fabrication of WO3 Nanoplates Thin Films with Dominant Crystal Facet of (002) for Water Splitting

Single crystalline orthorhombic phase tungsten trioxide monohydrate (O-WO3.H2O, space group: Pmnb) nanoplates with a clear morphology and uniform size distribution have been synthesized by the hydrothermal method and fabricated on the surface of fluorine doped tin oxide (FTO) coated glass substrates with selective exposure of the crystal facet by the finger rubbing method. The rubbing method can easily arrange the O-WO3.H2O nanoplates along the (020) facet on the FTO substrate. The O-WO3.H2O nanoplate can be converted to monoclinic phase WO3 (gamma-WO3, space group: P21/n) with dominant crystal facet of (002) without destroying the plate structure. Crystal morphologies, structures, and components of the powders and films have been determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Raman, X-ray photoelectron spectroscopy, etc. The band gap energies of the O-WO3.H2O and gamma-WO3 nanoplates were determined as ca. 2.26 and 2.49 eV, respectively. Photoelectrochemical properties of the films with (002) dominant crystal facet have also been checked for discussion of further application in water oxidation. The advantage of (002) facet dominant film was investigated by comparing to one spin-coated gamma-WO3 thin film with the same thickness via photoelectrochemical characterizations such as photocurrent, incident photon to current efficiency, and electrochemical impedance spectroscopy.